Turbine blades, such as in particular guide vanes or rotor blades of gas turbines, often have outwardly open cavities. These can reach deep into the blade interior, as is the case for example for cavities for passing cooling air through the turbine blade, or only slightly into the blade interior, as is the case for example for the depression of a crown formed at the blade tip. Cavities of this type are kept open during casting of the turbine blade by means of casting cores which are removed chemically or thermally from the cavity, via an outward opening of the cavity, once the material from which the turbine blade is made has solidified. In that context, the exact position of the casting cores during the casting process is hugely significant for the thicknesses of the turbine blade walls and, in the case of a crown, for the position of the crown base in the tip of the turbine blade.
In the context of qualification processes, turbine components are therefore tested once with respect to the radial position of the casting core. However, the destructive nature of the testing means that the tested turbine blade can no longer be used. This limits the frequency with which such quality testing can be carried out.
DE 10 2005 021 666A1 describes a method and a device for determining the position of configuration features, created by means of a core, in a lost-wax cast item. For this, core reference bases are incorporated into a core bead or flash section of the cast item, and serve as a precise reference system for locating the inner geometry and the position of structural features. On the basis of this reference system, it is then possible to carry out exact normalizing steps or precise chip-removing machining steps on the inner features created by means of a core. After the machining steps, the core bead or flash section is removed with the core reference base by chip-removing machining.
US 2012/0034097 A1 describes a turbine component having elements which make it easier to measure the wall thickness of the turbine components in the context of an inspection. The elements respectively comprise a planar inner inspection surface and a planar outer inspection surface that is aligned with the inner inspection surface. The inner inspection surface is produced by means of a casting core which has a surface for forming the inner inspection surface. In the context of the inspection, an inspection device such as an ultrasound probe, an eddy current probe or the like can transmit a signal into the mutually aligned inner and outer inspection surfaces in order to use the signal reflecting back to the inspection device to determine the wall thickness of the turbine component.
During mass production of turbine blades, it is possible for the position and/or orientation of the casting core(s), in particular the radial position of the casting core(s), to deviate, which becomes apparent only later when refurbishing worn turbine blades. A refurbishment process for a turbine blade generally involves chemically removing coatings, whereby an albeit small layer of the wall is also removed. Now, if during the casting process a casting core was not in its optimal position, this means that in some regions of the turbine blade the wall thickness is sub-optimal. This reduces the number of possible refurbishment processes for the gas turbine blade, which reduces the maximum service life of turbine blades and thus increases costs for a gas turbine plant. The wall thickness also influences heat transfer zones of a turbine blade.